but I can't see where it's going south and returning an array (or the string expression of an array). In other similar cases (outside the ifthenelse-decoder) sub-expression results are returned properly.

ifthenelse: if elsif(s?) else(?) endif
[download]

Since there may be more than one elsif, and else may be optional, these corresponding subexpressions return arrays (see Subrules from P::RD's docs). For example, in this production, $item[2] is an array of the semantic values of all the elsif nonterminals that were found at this point.

Two points:

• Use Data::Dumper instead of print statements to see what things are.
• I find the way you've written your grammar slightly unintuitive (although I admit I'm not really sure what you're trying to accomplish in your semantic actions). I would write it more like this:

use Parse::RecDescent;
use Data::Dumper;

my $parser = Parse::RecDescent->new(<<'END_GRAMMAR');
    expr:
        /\d+/      { $item[1] }
      | ifthenelse { $item[1] }
  
    ifthenelse:
        "if" expr "then" expr elsif(s?) else(?) "endif"
           { [ if => [@item[2,4]], @{$item[5]},
                     ref $item[6] ? $item[6][0] : undef ] }

    elsif:
        "elsif" expr "then" expr { [ @item[2,4] ] }

    else:
        "else" expr { $item[2] }
END_GRAMMAR

for (<DATA>) {
   print $_;
   print Dumper $parser->expr($_);
}

__DATA__
if 1 then 2 else 3 endif
if 1 then 2 endif
if 1 then 2 elsif 3 then 4 elsif 5 then 6 endif
if 1 then 2 elsif 3 then 4 else 5 endif
[download]

This returns an array that is structured like this:

[ "if", [ cond1, result1 ],   # first "if" condition/result expression
+ pair
        [ cond2, result2 ],   # 0 or more "elsif" expression pairs
        ...
        otherwise ]           # else branch expression, or undef if no
+ne
[download]

This data structure is fairly uniform, and should be easy to evaluate for whatever it is you're doing.

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